Typically, a multi-cylinder internal combustion engine of this type has a valve drive mechanism equipped with as many hydraulic valve lash adjusters as there are valves in the engine. To lubricate the hydraulic valve lash adjusters, a cylinder head of the engine, in which the hydraulic valve lash adjusters are located, is formed with a lubrication oil passageway or gallery in a wall thereof. Lubrication oil is generally introduced into a main oil passageway or gallery first, and then into a lubrication oil passageway or gallery for lubricating camshaft journals. After having lubricated the camshaft journals, the lubrication oil enters the valve lash adjuster lubrication oil gallery. Such a lubrication system is known from, for instance, Japanese Unexamined Utility Model Publication No. 59(1984)-107011.
In such a lubrication system in which lubrication oil is introduced into the lash adjuster lubrication oil gallery after having lubricated the camshaft journals, since the lubrication oil escapes or leaks past the camshaft journals, it is generally hard to maintain the lubrication oil at a sufficient oil pressure in the lash adjuster lubrication oil gallery. A reduction or drop in pressure occurs, in particular, at the valve lash adjuster or valve lash adjusters located on the downstream side of the lash adjuster lubrication oil gallery. This pressure drop allows air contamination of the lubricating oil in pressure chambers of the lash adjusters, so that the valve lash adjusters produce noise or encounter unstable operation.
For the purpose of preventing the lash adjuster from being subjected to a pressure drop, the above-described prior art lubrication system has been improved by providing a bypass oil passageway or gallery which connects the main oil gallery to an oil hole for the hydraulic valve lash adjuster and has a diaphragm and a valve, both disposed in the bypass passageway, so that when a reduction or drop in pressure occurs at the oil hole, the valve is forced to open in order to develop a sufficient oil pressure for the valve lash adjuster. This lubrication method is, however, apt to result in structurally complex lubrication system.
To eliminate adverse effects of the escape of lubrication oil at the camshaft journals to the hydraulic valve lash adjusters, the lubrication system could be organized by independently connecting a camshaft journal lubrication oil gallery and a valve lash adjuster lubrication oil gallery to a main oil gallery by way of separate lubrication oil passageways. However, because the lubrication oil escapes at the camshaft journals, even with the separation of the connecting oil passageways, it is still hard to provide the hydraulic valve lash adjusters with an oil pressure sufficient to eliminate adverse effects.